[media] mceusb: move check earlier to make smatch happy

Smatch complains that "cmdbuf[cmdcount - length]" might go past the end
of the array.  It's an easy warning to silence by moving the limit
check earlier.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/rc/mceusb.c

@@ -828,16 +828,16 @@ static int mceusb_tx_ir(struct rc_dev *dev, unsigned *txbuf, unsigned count)
 			 (txbuf[i] -= MCE_MAX_PULSE_LENGTH));
 	}
 
-	/* Fix packet length in last header */
-	length = cmdcount % MCE_CODE_LENGTH;
-	cmdbuf[cmdcount - length] -= MCE_CODE_LENGTH - length;
-
 	/* Check if we have room for the empty packet at the end */
 	if (cmdcount >= MCE_CMDBUF_SIZE) {
 		ret = -EINVAL;
 		goto out;
 	}
 
+	/* Fix packet length in last header */
+	length = cmdcount % MCE_CODE_LENGTH;
+	cmdbuf[cmdcount - length] -= MCE_CODE_LENGTH - length;
+
 	/* All mce commands end with an empty packet (0x80) */
 	cmdbuf[cmdcount++] = MCE_IRDATA_TRAILER;